usb: gadget: printer: remove unused and empty printer_unbind

The unbind() method is optional is usb_composite_driver.
In this particular driver the method does nothing so it can be removed.
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/legacy/printer.c

@@ -1288,11 +1288,6 @@ static int __init printer_bind_config(struct usb_configuration *c)
 	return status;
 }
 
-static int printer_unbind(struct usb_composite_dev *cdev)
-{
-	return 0;
-}
-
 static int __init printer_bind(struct usb_composite_dev *cdev)
 {
 	int ret;
@@ -1317,7 +1312,6 @@ static __refdata struct usb_composite_driver printer_driver = {
 	.strings        = dev_strings,
 	.max_speed      = USB_SPEED_SUPER,
 	.bind		= printer_bind,
-	.unbind		= printer_unbind,
 };
 
 static int __init